A sustained increase in crop yield, e.g., in wheat and rice has been achieved in the past few decades. This increase is partly attributed to the use of fertilizers and pesticides as well as the introduction of semi-dominant dwarfing mutations which reduce plant height. Taller plants are more likely to lodge in response to heavy rainfall or wind, and the heavier inflorescences of high-yield elite breeds also make them more susceptible to lodging. In contrast, crops with a shorter stature are more resistant to lodging. Moreover, dwarf and semi-dwarf traits can also allow higher planting densities and help improve crop harvest index and nitrogen response. The introduction of dwarf varieties of wheat and rice served as a cornerstone of the so-called “Green revolution” of the late 20th century.
Maize (Zea mays L.), a member of the Gramineae genus, provides cylindrical stalks similar to those from other grasses. The maize stalks are thick and spongy inside and divided into parts called internodes and nodes. The number of nodes ranges from between 8 to 40 depending on the variety and growing conditions. Commercial hybrid maize normally grows to a height of typically more than 2 meters with each plant having either one or two ears. The ear normally grows about one-third of the way up the plant or about three feet from the ground. Consequently a maize plant, while providing a large ear in addition to a substantial leaf and stalk structure, can have a considerable mechanical stability problem. Reducing the height of a maize plant can improve the mechanical stability of the plant.
More than 40 monogenic dwarfing mutants have been described in maize. A majority of these mutants lead to great reductions in grain yield and, consequently, they have not been used to enhance crop yield in germplasm that is sensible to lodging. Therefore, an important goal in corn breeding is to identify and use dwarf or semi-dwarf mutations which confer a short stature without severely impacting other organs, especially reproductive organs (e.g., ears).
In maize, brachytic mutants show a short stature due to a shortening of the internode length without a corresponding reduction in the number of internodes or the number and size of other organs, including the leaves, ear and tassel. See Kempton J. Hered. 11:111-115(1920); Pilu et al., Molecular Breeding, 20:83-91(2007). Three brachytic mutants have been isolated in maize to date: brachytic1 (br1), brachytic2 (br2) and brachytic3 (br3). Both br1 and br3 mutations cause a reduction in corn plant height which has been thought too severe for commercial exploitation due to potential impacts over yield. In contrast, the br2 mutant has particular agronomic potential because of the shortening of the internodes of the lower stalk with no obvious reduction in other plant organs. In addition, br2 lines exhibit an unusual stalk strength and tolerance to wind lodging, while the leaves are often darker and persist longer in the active green than those of the wild-type plants. The br2 phenotype is insensitive to treatment with Gibberellins, auxins, brassinosteroids and cytokinins, suggesting that the biosynthesis of these hormones is not modified by the br2 mutation.
Multani et al. identified the genomic sequence of the Br2 gene and deposited it under GenBank Accession No. AY366085. See Science, 302(5642)81-84 (2003). Br2 was annotated to encode a putative protein similar to adenosine triphosphate (ATP)-binding cassette transporters of the multidrug resistant (MDR) class of P-glycoproteins (PGPs). Pilu et al. reported a br2-23 allele having an 8-bp deletion in the 3′ end of the Br2 gene and claimed a direct relationship between this deletion and the brachytic phenotype in their br2-23 plants. See Pilu et al., Molecular Breeding, 20:83-91(2007). Nevertheless, the use of brachytic mutations in corn has not been exploited commercially partly because of the severity of the available brachytic mutant alleles.
There is a need in corn breeding to identify corn germplasm that provides novel and commercially relevant brachytic mutant alleles, e.g., those conferring an intermediate brachytic phenotype and maintaining or improving kernel yield. There is also a need to develop polymorphic markers for monitoring and introgressing novel brachytic mutant alleles, and further develop agronomically elite corn lines comprising a brachytic trait for enhancing plant productivity.